It is known that charges are formed on the surface of dielectric films when the films are rubbed or reeled or when the films are manufactured by previously known continuous manufacturing methods. The electric charges thus produced may generate electrostatic forces and the films may e.g. adhere to different surfaces. However, such electrostatic forces of attraction are very weak and the charges can easily disappear e.g. due to humidity, so the action of these forces may be of a very short duration. When this is the case, the films will not set straight on a surface and they are easily detached e.g. by air current.
Previously known are also electrets produced from solid material in which charges are generated in the film surface structure by subjecting it to a DC corona treatment. Such electret films can be utilized for example in various transducer and sensor applications, such as e.g. the electret microphone.
Producing a powerful charge penetrating deep into a solid film material is very difficult because it is not possible to increase the surface potential of such film material to a sufficiently high level during the corona treatment (charging) because of the leakage current flowing through the film. A leakage current flows because in solid material the resistance of the material is reduced as a function of voltage.
A further known method is to manufacture e.g. from polypropylene a bubble film which contains small, preferably flat gas bubbles placed in an overlapping arrangement inside the film. Due to the whiteness of the film resulting from the bubble structure, such film is an excellent material for printing. This kind of white film is generally used e.g. as packaging material.
In a film containing flat gas bubbles, resistivity is not reduced in the gas bubbles and the leakage current is not increased with voltage. Therefore, it is possible during charging to achieve a high electric field, e.g. in the range 100-200 MV/m, which in turn produces partial discharges in the bubbles contained in the film.